Microphone construction

ABSTRACT

A novel construction method is provided for a linear extended capacitor electret foil construction microphone with a construction technique providing an infinite number of small capacitors in an equidistribution arrangement, for providing maximum sensitivity for given field strengths, and to provide an in situ method of microphone fabrication wherein the microphone is completely assembled and run through each heat cycling polarization to produce a finished thermo electret element.

mted States Patent 1191 1111 3,821,491 Whetstone et al. June 2, 1974MICROPHONE CONSTRUCTION 3,373,251 3/1968 566161 179/111 R 3,474,l97l0/l969 K kl t l79/l I] E ,1 [75] Inventors Albert wheistmei southPort3,663,768 5/1972 143131626 6121 179/111 E Conn.; Samuel Fine, New Clty,N .Y.; Robert Davis, Prospect, Conn. FOREIGN PATENTS OR APPLICATIONS[73] Assigneez Amperex Electronic Corporation, 832,276 4/1960 GreatBritain 179/111 E Hicksvine, Long Island, Great Britain i l R [22]Filed: May 1 Primary Examiner-Kathleen H. Claffy 2 App[ 253,491Assistant Examiner-Thomas L. Kundert [52] US. Cl. 179/111 E, 307/88 ET,29/594 ABSTRACT [51] Int. Cl H04! 19/00 A novel Construction method isprovided for a near [58] Field of Search 79/111 E, extended capacitorelectret foil construction micro- R; 307/88 ET; 29/ 594 phone with aconstruction technique providing an infnite number of small capacitorsin an equidistribution [56] References cued arrangement, for providingmaximum sensitivity for UNITED STATES PATENTS given field strengths, andto provide an in situ method 1,644,387 10/1927 Kyle 179/111 R ofmicrophone fabrication wherein the microphone is 1,983,377 12/1934Kellogg 179/111 R completely assembled and run through each heat cy-2,615,994 10/1952 Lindenberg et al 179/111 R cling polarization toproduce a finished thermo elec- 3,008,013 1 H1961 Williamson 61 al.179/] R [yet element 3,300,585 l/l967 Reedyk et al. l79/lll R 3,328,6536/1967 Wolf, Jr. 179/111 R 4 Claims, 5 Drawing Figures PAIENTEBmza mm A382L491 saw 1 0r 2 GROUND TAB SECURING END SECURING TAB END SECURINGADHESIVE PATENTED JUN 2 8 I974 SHEETEUFE Fig.4

MHCRDPHONE CONSTRUCTION This invention relates to electro statictransducers and more particularly to an improved method of constructingan electro static transducer employing the thermo electret foilconstruction. I

Condenser or electro static microphones of the class described operateby means of a relative vibration of a pair of spaced electrodes.Efficient condenser type microphones having high capacitance are madepossible by the use of a thin metalized dielectric film placed next to asolid back plate with a minimum air gap. The metal side of thedielectric forms one plate of the capacitor while the solid back formsthe other. in this situation a DC bias is required to operate the soliddielectric microphone. in some dielectric materials, however, thereexists a small electro static polarization which is created during themanufacturing process and can have a long lifetime. This polarization,which makes the foil electret, can aid or oppose applied DC voltage.Prepolarization' of .foils has certain distinct advantages and can beaccomplished by the formation of a thermoelectret process wherein adielectric foil can be heated to approximately 150C and exposed to ahigh DC field. The foil is then allowed to cool in the DC field and astrong polarization of the foil results. This polarization eliminatesthe need of an external DC bias which is necessary in other types ofcondenser microphones. Construction of such devices in the past hasproved relatively difficult in view of the high precision andmanufacture necessary in order to avoid foreign particles in the spacingbetween the movable plate and the back plate. One proposed solution hasbeen to add additional dielectric layers between the movable and theback fixed plate, however, as a result the spacing between the movableplate and the fixed plate is relatively large and the transducercapacitance relatively low. Another proposed solution is to provide aporous back plate formed of a suitable texture and porosity so as toavoid an accumulation of air bubbles beneath the diaphragm and yet toprovide a plurality of contact points between the movable plate and thefixed plate. The porous back plate in such case has been formed by theuse of sintered porous materials, wire mesh screens, metallic foams,metalized plastic, and the like. Such manufacture has been relativelyuneconomical as well as difficult in production. It is further desirablein the forming of an electret type microphone to provide a selfshielding, self polarized construction of desired shape adaptable to beemployed with an integrated amplifier. Each of the sensors describedabove have cer tain principles of operation and construction. Extendedsensors are essential linear extended capacitors, flat or cylindrical inthe sense that the ratio length to height of the sensitive surface isusually, although not always, a relatively large number. In this case,one capacitor plate is at a potential above or below ground and is notmovable. The other capacitor plate, preferably in the form of a thinplastic film metalized on one surface, is capable of vibratory motioninduced by the absorption of sonic energy. This plate is at groundpotential. It is noted that the nonmovable plate from which a signal istaken, as through a series load resistor, is sandwiched between anothernonmovable plate at ground potential from which it is insulated, themicrophone frame itself, and the thin movable film which is also atground.

. constructable.

Therefore, it is the prime object of the present invention to provide anovel electret microphone construction which provides for a superiormicrophone than has heretofor been available.

It is a further object of the present invention to provide a microphoneconstruction employing a shielded sandwich structure.

It is a still further object of the present invention to provide a novelmethod of manufacturing the stationary plate forming the sub-surface ofthe microphone to provide an equidistribution of an infinite number ofsmall (1 capacitors thereby improving the output level relative tonoise.

It is another object of the present invention to provide a thermoelectric microphone without the necessity of preheating the electretfoil.

It is another object of the present invention to permit the constructionof extremely long microphones by providing microphone sensors insegments with a pre-. amplifier integrally connected to each segment.

The foregoing objects are accomplished by means of a microphoneconstruction which employs the use of a I mounted non-movable plateaffixed to the microphone frame, which is used to supply the signaltransduced by the microphone. This nonmovable plate is sandwichedbetween the microphone frame and the thin movable film. Since the thinmovable film is at ground potential and the microphone frame is atground potential, the movable film and thus the output of the microphonesystem is excellently shielded from any surrounding factors. Themagnitude of the voltage output of a vibrating capacitor, such as isdescribed above, can be calculated. Assuming a large load resistor sothat the charge remains constant, then v q/c and c KA/d, giving v qd/KAwhere v is voltage, c is capacitance, q is charge, K is the di-electricconstant, A is area, d is spacing, and A is a partial derivative.Differentiating, dv q Ad/KA and Av/v Ad/d. Since Ad will be very small,Av/v is maximized if d is small but not zero. To take advantage of theforgoing, the present invention employs the technique of knurling orsanding the outer surface of the stationary plate affixed to themicrophone frame which faces the thin film in order to form anequidistribution of an infinite number of small d capacitors, yieldingan output estimated at three to five times greater than if the movablefilm were placed directly on a smooth fixed plate. The nature of thefilm used as the vibrating plate is also important in order to maximizeAd. To this end, the film must not be too thin or thick nor too stiffnot pliant, so that it vibrates with the maximum amplitude in responseto impinging sonic shock waves.

Prior art techniques also employ the practice of placing the film andapplying an external polarizing voltage as a source of charge on thecapacitor plates. This required an extremely controlled supply, and careto prevent ohmic leakage with attendant high noise level. This methodwas changed when it was discovered that many film materials could bepermanently polarized by means of a suitable heating cycle incombination with a suitable polarizing voltage applied during thecooling period. Such a film, referred to as a thermo electret, retaineda specific half life charge retention ranging from weeks to hundreds ofyears. The present invention further employs the novel and new techniqueof an in situ method of microphone fabrication wherein the microphone iscompletely assembled and then placed through the heat cycling polarizingprocedure to produce a finished thermo electret sensor.

The foregoing description and stated objects will become more apparentfrom the following more detailed description of the construction andoperation of the microphone of the present invention wherein:

FIG. 11 shows a cross-section of a microphone in accordance with thepresent invention,

FIG. 2 shows an exploded isometric of a linear extended microphone,

FIG. 3 illustrates a cross-section of a cylindrical embodiment,

FIG. 4 a sectioned cylindrical unit, and

FIG. 5 shows the sectioned microphone and amplifier configuration.

Referring now to FIG. 1, a cross sectional embodiment of the microphonearrangement of the present invention is illustrated. As shown therein,an aluminum substrate 14 is provided with a first layer 16 attachedthereto. The layer 16 includes a copper segment 20 formed as part of afilm which includes an insulator backing 18. To the layer 16 is attacheda second layer 22 which consists of an aluminized polycarbonate filmhaving a layer of aluminum 24 forming the upper surface thereof and thepolycarbonate thin film insulating material 26 forming the lower surfacethereof. The insulting layer 18 may be any suitable insulating materialwith appropriate dielectric properties such as a glass loaded epoxysubstance. To the upper metalized layer 24 is attached a ground lead 28placing the upper layer 24 at a reference or ground potential. To thealuminum layer 14 is attached a ground lead 30 placing the aluminumlayer at the same reference or ground potential. The operative layer,the copper layer 20, is provided with a lead 32 providing the outputsignal therefrom. The copper layer 20 itself is provided with aplurality of surface spacings indicated generally as 34 which providethe plurality of equidistant spaced d length capacitors as describedabove. The technique for creating these areas 34 will be described infurther detail below.

Referring now to FIG. 2, an exploded isometric view of the microphoneconstruction is illustrated. As shown therein, an aluminum extrusion inthe shape of the ell bar 34 is provided. The ell bar is shown for thepurpose of making a flat linear extended microphone. It should be notedthat a cylindrical or other shape microphone can be formed as desired,as will be explained in further detail below. The construction techniqueremains the same as with the flat linear extended microphone. Thus, thealuminum extrusion 34 is designed to receive a copper clad epoxy film36. The copper surface 38 of the epoxy film is previously treated by anappropriate scoring process such as sand blasting, or knurling. Theeffect of sand blasting or knurling is to produce a series of randomlyspaced depressions within the copper surface which form an infiniteplurality of substantially equidistant spaced d capacitors as describedabove. The film 36 may be attached to the surface of the aluminumextrusion 34 by means of a suitable adhesive, such as epoxy.

Since the electret, the outer element, which is preferably an aluminizedmylar polycarbonate film 40, is hydroscopic in nature, it may bepreviously treated in an oven to eliminate any excess moisture which mayhave been formed prior to polarizing. Alternatively, moisture may beeliminated by use of a suitable desicant.

To the upper part of the assembled film and aluminum extrusion is placeda strip of tape 42 and a lower strip 44. To this tape, which is stickyon both sides, the aluminum polycarbonate material 40, is applied. Theouter surfaces are now trimmed to assure the aluminized mylar elementconforms to the surface of the aluminum extrusion 34 and the ground barflap 46 is wrapped around and underneath the surface of the aluminumextrusion 34 and makes contact directly thereto to provide a commonground reference potential for both the aluminum extrusion frame elementand the outer aluminum surface of the aluminized polycarbonate film 40.By making both the elements 40 and 34 at a common ground potential,adequate isolation of the internal copper element 38 is assured.

Next, electret polarization is effected by a thermal treatment. To thisend, the entire assembly is placed in a furnace. A high voltage ofapproximately 500 volts negative is applied to the bar 34 with referenceto the copper film 38. With the voltage on, the temperature of the ovenis raised to 150 at which point the oven is turned off. The oven is thenallowed to cool to with the voltage on. When the work reaches the roomtemperature, and the aluminized surface is polarized as evidenced byadherence of the foil to the copper film, significant to show thegrating detail of the scoring of the film, the voltage may be turnedoff.

As an alternative, a cylindrical embodiment may be employed. Thus,referring to FIGS. 3 and 4, an aluminum tube frame 48 is provided with afirst layer 50 of copper laminate plate and a second layer 52 of analuminized polycarbonate film. As shown, the copper layer 62 iselectrically separated from the aluminum tube 48 by insulating layer 64,and the aluminized layer 58 separated from the copper layer by aninsulator 60. Common ground is formed by electrically connecting thealuminized layer 58 to the frame 48. The techniques of applying theplate as well as thermally treating the film are precisely as describedabove in connection with the ell bar. With regard to this embodiment, asmall region of the film 52 is devoid of foil 54 to prevent arcing fromthe aluminized layer to the copper. A further overhang of the layer 52with respect to layer 50 to prevent arcing is shown in FIG. 3. Also, tofacilitate connection of the lead, the film 52 may overlap the film 50at one arcuate end, and underlap the film 50 at the other end.

As described above, in microphones of undue length, of eithercylindrical or flat configuration, the signal to noise ratios may beimproved by providing sectioning. Sectioning is accomplished bysplitting the copper coating of the layer 50 in a radial fashion at setdistance from the end of the frame to form a desired section length.

The laminate 50 is sectioned by split down through the conductivesurface 62 to, but not through, the insulation backing 64. The outsidelayer 52 need not be split since it forms a common ground reference forthe laminate segments.

During the formation of the microphone, where two segments of laminatematerial are epoxied to the frame in a side by side relationship, it hasbeen found that epoxy material inevitably leaks between the segments.When thermally treated, the epoxy becomes conductive, and forms aconductive path from aluminum to the copper, and thus inhibitspolarization during the thermal cycle. To prevent this occurrence, thisinvention employs the technique of providing a slit, made through thebutt area all the way down to the frame and thus removing the excessepoxy path.

Referring to FIG. 5, showing the rear of a linear flat surfacemicrophone, each microphone section 68 is coupled by virtue of anelectrical lead 70 to an amplifier 72. The output of each amplifier iscoupled through each subsequent amplifier for providing a common output.It has been found that this arrangement results in an outputcharacteristic equivalent to that caused by a single microphone section,and that this characteristic remains regardless of the length of themicrophone or the number of sections. Obviously, the cylindricalembodiment may employ the same configuration.

Alternatively, a summing amplifier may be employed with each amplifiersection 72 coupled thereto.

Amplifiers may be physically attached to the reverse side of the surfaceof the microphone which receives sound sensing and a lead attached tothe copper area directly. The lower end of the microphone may include aslight overhang of metalized film 56 to prevent arcing therein as well.As shown in FIG. 4, a sectional microphone is illustrated. By removingelements of the upper film, segments of the copper can be exposed toallow for a microphone amplifier connection to be made directly to thecopper.

In a typical physical embodiment, a cylindrical microphone may embrace acylinder of aluminum with a 2 inch outside diameter 36 inches longcovered with a 2 /2 inch wide 5 mil copper laminate. As described above,the copper is scored as by sand blasting and may be split into threeequal sections. The copper area is covered with a 2 inch wide film whichis formed with a capacity of about 10,000 picofarads for 12 inch lengthsin a manner similar to that described for a flat sensor. In acylindrical embodiment, the arcuate angular sweep is as much as 104, thepulse output in this case with a sound source 36 inches away is 2millivolts for each section. I

With faster response times, noise is less of a problem. The responsetime of the microphone output is markedly affected by the thickness,density, stiffness of the vibrating film. The following list of filmmaterial, and processing temperatures, is not intended to be exclusivebut merely illustrative of the different film materials and theirproperties. Also, variation within the temperature regions may berealized within the scope of the invention.

MATERIAL (FILM) PROCESSING TEMPERATURE Heat Cool $6 mil aluminized mylarI25C 60C mil aluminized polycarbonate 150C 75C A mil goldized polyimide150C 75C mil aluminized polypropylene l 10C 55"C mil aluminizedpolystryene C 35C 1 mil aluminized teflon 150C C mil aluminizedpolysulfonr C 75C While the invention has been described and shown withreference to certain preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention.

What is claimed isi l. A microphoneconstruction comprising:

an elongated conductive frame member;

a plurality of non-moveable plate sections affixed to and insulated fromsaid frame, each said plate section including a conductive surfacefacing away from said frame and having a multiplicity of distributedminute depressions therein;

a plurality of thin moveable electret insulating film sections disposedon the conductive surfaces of said plate sections, each of said filmsections having an outer conductive layer;

means for coupling each of said outer conductive layers to said frame;and

. means for coupling the plate sections in series to a common output.

2. A method of manufacturing a microphone on an elongated frame membercomprising the steps of:

applying a conductor-clad insulator to said frame, the

insulating side facing said frame;

sectioning the first applied layer into multiple sections;

applying an insulating layer having a thin conductive film on the outersurface thereof to said conductorclad insulator;

electrically coupling said sections in series;

applying a voltage between said conductive film and the conductivesections of said conductor-clad insulator; and

heating the entire assembly while said voltage is being applied.

3. The method as defined by claim 2 comprising the additional step ofremoving a multiplicity of particles from the conductive portion of saidconductor-clad insulator before the application of said insulating layerthereto.

4. The method as defined by claim 2 comprising the additional step ofelectrically coupling said conductive film to said frame beforeapplication of said voltage.

1. A microphone construction comprising: an elongated conductive framemember; a plurality of non-moveable plate sections affixed to andinsulated from said frame, each said plate section including aconductive surface facing away from said frame and having a multiplicityof distributed minute depressions therein; a plurality of thin moveableelectret insulating film sections disposed on the conductive surfaces ofsaid plate sections, each of said film sections having an outerconductive layer; means for coupling each of said outer conductivelayers to said frame; and means for coupling the plate sections inseries to a common output.
 2. A method of manufacturing a microphone onan elongated frame member comprising the steps of: applying aconductor-clad insulator to said frame, the insulating side facing saidframe; sectioning the first applied layer into multiple sections;applying an insulating layer having a thin conductive film on the outersurface thereof to said conductor-clad insulator; electrically couplingsaid sections in series; applying a voltaGe between said conductive filmand the conductive sections of said conductor-clad insulator; andheating the entire assembly while said voltage is being applied.
 3. Themethod as defined by claim 2 comprising the additional step of removinga multiplicity of particles from the conductive portion of saidconductor-clad insulator before the application of said insulating layerthereto.
 4. The method as defined by claim 2 comprising the additionalstep of electrically coupling said conductive film to said frame beforeapplication of said voltage.